1. Field of the Invention
The present invention generally relates to a method for diagnosing an abnormality of a sensor, and more particularly to an abnormality diagnosis method suitable for various sensors provided for an electronic control type engine.
2) Description of Related Arts
Generally, various sensors, such as a water temperature sensor and an air flow meter, are provided for an internal combustion engine equipped with a fuel injection control system. The fuel injection control system receives output signals generated and output by the sensors, and controls fuel injection on the basis of the received output signals. Such a fuel injection control can provide an amount of fuel injected suitable for the current condition of the engine, so that improved engine characteristics can be obtained.
When the sensors are normally working, the fuel injection control can be carried out accurately. On the other hand, if a failure (abnormal state) occurs in any one of the sensors, the fuel injection control will provide an incorrect fuel amount. For this reason, it is necessary to definitely detect an abnormality of each sensor in its early stage. For this purpose, the engine is equipped with a diagnostic system.
Japanese Laid-Open Patent Application No. 62-47528 discloses a method for diagnosing an abnormality of temperature sensors. The proposed method uses a temperature switch which carries out a switching operation at a predetermined temperature. A correction value is calculated on the basis of a numerical relationship between the output value of the temperature sensor obtained when the temperature switch operates and a predetermined design value. The output value of the temperature sensor is corrected in accordance with the calculated correction value.
Japanese Laid-Open Utility Model Application No. 63-70019 discloses a method for detecting an abnormality of a connector part of a sensor, such as a defect in connection, by determining whether or not a variation in the output value of the sensor exceeds a predetermined value.
However, the above-mentioned proposed methods have the following disadvantages. The method disclosed in Japanese Laid-Open Patent Application No. 62-47528 needs the temperature switch specifically used for abnormality diagnosis. This increases the production cost. The method disclosed in Japanese Laid-Open Utility Model Application No. 63-70019 can detect only a failure which results from an instant change in the output signal of the sensor, which will be caused by the occurrence of a serious defect in connection. If an oxide film is formed on a connecting portion of the connector part of the sensor, the sensor characteristic will gradually deteriorate. In this case, the proposed method cannot detect such a deterioration with age.